Chapter 1 of
Publication 590,
Individual Retirement Arrangements (IRAs), gives a complete explanation of how to calculate this amount.

Generally,

The minimum distribution for each year is computed by dividing the IRA account balance at the close of business December 31 of the preceding year by the life
expectancy.

The life expectancy is determined by using one of three tables found in Publication
590:

Table I is used by a designated beneficiary who, after the death of the IRA owner, is the sole
beneficiary.

Table II is generally for use by an IRA owner whose spouse is more than 10 years younger and is the sole designated
beneficiary.

Table III is generally for use by an unmarried IRA owner, an IRA owner whose spouse is the sole designated beneficiary and not more than 10 years younger, and an IRA owner whose spouse is not the sole
beneficiary.